1. Field of the Invention
The present invention relates to composite torque tubes and the method for forming the same.
2. The Prior Art
Composite tubes have long been recognized for their light weight and torque transmitting capabilities. To realize the full benefit of the torque carrying capacity it is necessary that the remaining components of the torque transmitting string be especially strong.
The inherent difficulty in forming a high integrity interface between a composite tube and the adjoining surfaces has long been recognized. It has been common practice to form a joint with a mutual fitting or sleeve having a shell with a single surface abutting the wall of the composite torque pipe. This arrangement provides a single interface for bonding, sometimes referred to as a single shear lap joint, a method of forming a joint of this type is shown in U.S. Patent Appl. No. 2005/013783 to Williams. These single lap joints fail to provide the integrity necessary to carry high torque loads without failure.
In the past, fitting assemblies with variously opposing surface geometries have been proposed to effect a secure capture of the composite end of a torque pipe within the fitting. Some examples of making, such end fittings include those taught in U.S. Pat. Nos. 4,421,497 to Fiderman; 5,233,737 to Policelli; 4,810,010 to Jones; 6,315,002 to Antal et al.; and others. While suitable for the purposes intended each of the foregoing assemblies include threaded or otherwise releasably engaged parts clamping or compressing the composite between each other with inherently uneven load concentrations resulting in highly uneven shear stresses. This uneven load distribution between adjacent parts, of course, results in correspondingly uneven local strain deformations when exposed to the various high loadings in the course of use. There is therefore an inherent incidence of local bond separation between the composite itself and the adjoining fitting surface, with some consequence for failure.
Artisans have recognized that the high torque loads applied to drive shafts cannot be adequately carried by previous proposals for wrapping filament bundles around circumferential grooves on a sleeve periphery and proposed a method for employing a tubular sleeve with longitudinal knurls, U.S. Pat. No. 4,238,539 to Yates. Devices by this method are expensive to make and fail to provide the requisite load carrying capability for many high torque applications.
Efforts to enhance joint strength have led to proposals that the interference between an end fitting and pipe be splined or grooved longitudinally or circumferentially for receipt as a bond. Approaches of this type are shown in U.S. Pat. Nos. 4,830,409 to Freeman, 4,952,195 to Taylor and 5,601,494 to Duggar. Joints made by these methods fail to provide smooth, uniform interfaces to enhance the bond strength and distribute stress uniformly over the joint interface.
Alternatively, end fitting assemblies have been proposed in which radial pins or other radial fasteners are added to the assembly, as exemplified by the teachings of U.S. Pat. Nos. 5,332,049 to Tew; 5,288,109 to Auberon et al.; 5,443,099 to Chaussepied et al.; and others. Once again, while a change is realized from these radial interconnections the essentially separated nature of a single metal to composite surface interface is also susceptible to uneven load transfer with the consequent local separations an inherent possibility. For example, the '049 patent to Tew appears to disclose a single metal-composite interface held together by radial pins and an adhesive bond which may suffer from disparate torsional forces. Tew appears to propose a cylindrical outer protective sheath drawn over the pipe and lacking a tapered surface interface and suffers the shortcoming that, the coupling itself fails to provide a high strength joint capable of carrying the high torsional force necessary to withstand the loads of both extended reach applications and short radius.
It can be seen then that a need exists for a lightweight and durable structure capable of withstanding the rigors of deep and directional drilling that is also capable of carrying a protected signal down a pipe string length.